With the scarcity of worldwide petroleum resources, internal combustion engines fueled by conventional and unconventional natural gas (short for gas engine) are being used increasingly because of their characteristics of cleanness, high efficiency, low pollution and huge potential of gas resources. In particular, a gas combustion engine for large-scale power generation together with its waste heat recovering system often takes as a set of independent system for supplying energy to the buildings. Many developed countries pay attention to this system and called it as “second generation energy system”, which is widely used in the field of energy supply for buildings. The gas engine has various waste heat sources, each with different grades. The main heat source is engine exhaust, which temperature could up to 600° C.; followed by the waste heat of jacket water, which temperature ranges from about 75° C. to 85° C.; furthermore, the temperature of the charge air in a turbocharged combustion engine can be more than 100° C. The heat amount and quality of these main heat sources vary significantly, and the temperature of waste heat is greatly reduced after recovering, owing to large temperature difference waste heat having large span of energy quality. However, any existing waste heat recovering method can only efficiently recycle the heat within a certain energy quality, therefore, a single waste heat recovering method may not make full use of the waste heat of the gas engine.
Therefore, it is required to create a method combining multiple waste heat recovering methods to make full use of waste heat in the gas engine on the basis of the principle of stepwise utilization of energy and considering the different energy quality requirements for building energy (e.g. a building has various requirements to the energy quality, such as requiring high-grade energy for electricity generation and requiring medium/low-grade energy for cooling or heating).